In recent years, following an increase in utilization of various electrical equipment and electronic application equipment, electromagnetic interference (EMI) increases rapidly. It is pointed out that EMI not only becomes a cause of malfunction or interference of electronic or electric appliances but also gives health interference to an operator of such a device. For that reason, in electronic or electric appliances, it is required that the intensity of electromagnetic wave discharge is controlled within the standards or regulations.
For a countermeasure against the EMI, electromagnetic waves must be shielded. In order to achieve this, it is self-explanatory to utilize such properties of a metal that it does not penetrate electromagnetic waves therethrough. For example, there are employed a method of forming a casing by a metal material or a highly conductive material; a method of inserting a metal plate between a circuit board and a circuit board; and a method of covering a cable by a metallic foil. However, in CRT, PDP, etc., since an operator must recognize letters or the like displayed on a screen, the transparency of the display front face is required. According to all of the foregoing methods, the display front face often becomes opaque. Thus, these methods are often inadequate as an electromagnetic shielding method to be applied to display appliances.
In addition, since PDP emits large quantities of electromagnetic waves as compared with CRT, etc., it is required to have a stronger electromagnetic shielding ability. For example, in an optically transparent electromagnetic shielding material for CRT, if its surface resistivity value is not more than approximately 300 Ω/sq, there is no hindrance. On the other hand, in an optically transparent electromagnetic shielding material for PDP, a surface resistivity value of not more than 2.5 Ω/sq is required. In order to meet such a requirement of high conductivity, a method of applying an opening pattern by a photolithographic measure to a metal foil with sufficient conductivity is employed.
However, for example, a copper foil which becomes a material for forming the foregoing shielding layer has a metallic luster. Accordingly, there is involved a problem that not only light from the outside of the panel is reflected, whereby the contrast of the screen is deteriorated, but also the reflected color of the copper foil is viewed. Furthermore, there is involved a problem that light emitted within the screen is reflected, too, whereby the image display quality of the display panel is deteriorated.
In order to effectively prevent all of the reflection of emitted light within the screen and incident light from the outside and leakage of electromagnetic waves, it is known that blackening treatment of a shielding film such as a copper foil is effective.
In particular, as a blackening treated film of copper foil for plasma display panel, it is desired that the surface of the blackening treated film is uniform and free from or extremely low in the generation of streak unevenness; that etching properties are good; that discoloration with time is small; and that the blackening treated layer is hardly peeled away (excellent in adhesion).
Patent Document 1 discloses an electromagnetic shielding film for plasma display panel, which is provided with a black nickel plated layer containing from 500 to 20,000 μg/dm2 (from 0.05 to 2 g/m2) of zinc and from 100 to 500 μg/dm2 (from 0.01 to 0.05 g/m2) on a surface of a copper foil and describes that a surface of a blackening treated film is uniform and small in streak unevenness. Patent Document 2 discloses an electromagnetic shielding plate which is configured to stack successively a blackening layer, a conductive pattern layer and a blackening layer in this order. Though the electromagnetic shielding has a complicated configuration, the generation of moiré is prevented by stacking the blackening layers. Also, Patent Document 3 discloses a shielding material in which visibility is improved by blackening both surfaces and side faces of a metal layer pattern to control reflection of both outgoing light and incident light. In addition, Patent Document 4 discloses an electromagnetic shielding filter having an improved contrast ratio by controlling reflected light by a blackening layer formed on a surface of a conductive pattern. All of the blackening layers disclosed in Patent Documents 2 to 4 are formed by a mixed layer of nickel and zinc.
Also, Patent Document 5 mentions that when plating with zinc or a zinc alloy is applied onto a glossy surface of a copper foil for printed circuit and the surface is then treated with benzotriazole or a benzotriazole derivative, the adhesion of a resist during photolithography is improved. This patent document is characterized in that the zinc used herein is extremely thinly formed in a deposition amount of from 100 to 500 μg/dm2 (from 0.01 to 0.05 g/m2). If it is intended to apply this as an electromagnetic shielding film for PDP, discoloration with time is large, and therefore, such was problematic.
On the other hand, the development of technologies for forming a metallic conductive thin film on an insulator film with high productivity, such as an electromagnetic shielding film which is used in a flexible wiring plate to be used in electronic appliances or a plasma display is desired.
For example, Patent Document 6 discloses a method of manufacturing an electromagnetic shielding film by exposing and developing a photosensitive material containing a silver salt and further applying physical development or plating treatment to the developed silver. According to Patent Document 6 it is described that as compared with other systems, a fine line pattern can be precisely formed, thereby obtaining an excellent electromagnetic shielding film such that it is high in transparency and that it is able to be mass-produced inexpensively.
[Patent Document 1] JP-A-2004-145063
[Patent Document 2] JP-A-11-266095
[Patent Document 3] JP-A-2002-9484
[Patent Document 4] JP-A-2004-320025
[Patent Document 5] JP-A-6-85417
[Patent Document 6] JP-A-2004-221564